In the past, a semiconductor thin film such as polycrystalline silicon (poly-Si) was widely used for thin film transistors (TFT) or solar cells. In particular, poly-Si TFT having high carrier mobility can be manufactured on a transparent insulating substrate such as a glass substrate. Using such characteristics, poly-Si TFT is widely used for, for example, a switching element that constitutes a pixel circuit such as a liquid crystal display apparatus, a liquid crystal projector, or an organic EL display apparatus, or a circuit element of a liquid crystal driver.
As a method of manufacturing a high-performance TFT on a glass substrate, a manufacturing method termed a “high-temperature process” is generally used. Among TFT-manufacturing processes, a process in which the peak temperature is approximately 1000° C. is generally termed the “high-temperature process.” The characteristics of the high-temperature process include a capability of forming a relatively favorable polycrystalline silicon film using the solid-phase growth of silicon, a capability of obtaining a favorable gate insulating layer using thermal oxidation of silicon, and a capability of forming an interface between pure polycrystalline silicon and the gate insulating layer. In the high-temperature process, a high-performance TFT having high mobility and high reliability can be stably manufactured due to the above characteristics.
On the other hand, the high-temperature process is a process for crystallizing a silicon film using solid-phase growth. Therefore, in the high-temperature process, it is necessary to perform a treatment for a long period of time of approximately 48 hours. Since the above process takes a significantly long process, in order to increase the throughput of the process, a number of thermal treatment furnaces are inevitably required and it is difficult to reduce the cost. Additionally, in order to perform the high-temperature process, A quartz glass has to be used as the highly heat-resistant insulating substrate. Therefore, the cost for a substrate is high, which does not make an increase in the size suitable.
Meanwhile, among TFT-manufacturing processes, a process in which a poly-Si TFT is manufactured on a relatively cheap heat-resistant glass substrate under a temperature environment in which the peak temperature is approximately 600° C. or lower is generally termed the “low-temperature process.” In the low-temperature process, a laser crystallization technique in which a silicon film is crystallized using a pulse laser having an extremely short oscillation time is widely used. Laser crystallization refers to a technique using a property of a silicon thin film in a process in which the silicon thin film on a substrate is irradiated with a high-output pulse laser ray so as to be instantly melted, and crystallized in the process of solidifying.
In order to overcome the problems of the limitation on the size of the substrate and the large apparatus costs, a crystallization technique termed a “thermal plasma jet crystallization method” is being studied (for example, refer to NPL 1). Hereinafter, this thermal plasma jet crystallization method will be described simply.
A tungsten (W) anode and a water-cooled copper (Cu) cathode are disposed opposite to each other, and an arc discharge is generated between both electrodes when a DC voltage is applied. When argon gas is made to flow between the electrodes under atmospheric pressure, thermal plasma is ejected from an ejection hole opened in the copper cathode. The thermal plasma refers to thermal equilibrium plasma, and is an ultra high-temperature heat source in which ions, electrons, neutral atoms, and the like have substantially the same temperature which is approximately 10000 K. Due to the above fact, the thermal plasma can easily heat matter to a high temperature. An a-Si film can be crystallized by scanning the entire surface of a substrate having the a-Si film deposited thereon with ultra high-temperature thermal plasma at a high rate.
As such, in the thermal plasma jet crystallization method, the apparatus configuration is extremely simple, and crystallization is achieved under atmospheric pressure in the process. In addition, it is not necessary to cover the apparatus with an expensive member such as a sealed chamber, and a significant decrease in the apparatus costs can be expected. In addition, since utilities necessary for crystallization are argon gas, electric power, and cooling water, the crystallization technique also has a low running cost.
FIG. 17 is a schematic diagram for explaining a method of crystallizing a semiconductor film using the thermal plasma jet crystallization method. In FIG. 17, thermal plasma-generating apparatus 31 has anode 32 and cathode 33 which is disposed opposite to anode 32 with a predetermined distance therebetween. Anode 32 is constituted by, for example, a conductor such as tungsten. Cathode 33 is constituted by, for example, a conductor such as copper. In addition, cathode 33 is formed to be hollow, and is configured to allow water to pass through the hollow portion so as to make cooling possible.
Ejection hole (nozzle) 34 is provided in cathode 33. When a direct (DC) voltage is applied between anode 32 and cathode 33, an arc discharge is generated between both electrodes. When gas such as argon gas is made to flow between anode 32 and cathode 33 under atmospheric pressure in the above state, it is possible to eject thermal plasma 35 from ejection hole 34.
The thermal plasma can be used for a thermal treatment for crystallization of a semiconductor film. Specifically, semiconductor film 37 (for example, an amorphous silicon film) is formed on substrate 36, and thermal plasma (thermal plasma jet) 35 is made to irradiate semiconductor film 37. Thermal plasma 35 is made to irradiate semiconductor film 37 while relatively moving along the first axis (the horizontal direction in the example shown in the drawing) that is parallel to the surface of semiconductor film 37. That is, thermal plasma 35 is made to irradiate semiconductor film 37 while scanning in the first axial direction. Using the scanning of thermal plasma 35, semiconductor film 37 is heated due to a high temperature of thermal plasma 35, and crystallized semiconductor film 38 (a polysilicon film in the present example) is obtained (for example, refer to PTL 1).
FIG. 18 is a conceptual view showing the relationship between the depth from the outermost surface and the temperature of semiconductor film 37 which is irradiated with thermal plasma 35. As shown in FIG. 18, it is possible to treat only the vicinity of the surface of semiconductor film 37 at a high temperature by moving the thermal plasma 35 at a high rate. Since the regions irradiated with thermal plasma 35 are rapidly cool after being irradiated, the vicinity of the surface remains at a high temperature for an extremely short period of time.
The thermal plasma is generally made to irradiate dotted regions. The thermal plasma is maintained using thermionic emission from anode 32. The thermionic emission becomes more active at locations having a high plasma density. That is, the arc discharge is generated intensely at one point in the anode, and the thermal plasma is generated in dotted regions. In this way, the thermal plasma being selectively generated in dotted lines refers to a positive feedback being applied.
In a case in which it is necessary to uniformly crystallize a semiconductor film formed on a tabular base material using thermal plasma, it is necessary to scan dotted thermal plasma across the entire base material multiple times. In order to build a process in which the number of times of scanning is reduced so that a treatment can be performed within a shorter period of time, it is effective to widen the irradiation region of the thermal plasma. Therefore, thus far, techniques that generate thermal plasma in a large area have been studied.
For example, a plasma torch including a plasma jet nozzle and a broadened gas nozzle (for example, refer to PTL 2), and a method is disclosed in which a plasma jet irradiated from the plasma jet nozzle is widened by gas ejected from the broadened gas nozzle.
Alternatively, a plasma torch is disclosed in which an opening portion of a plasma nozzle is inclined with respect to a core of a nozzle path (for example, refer to PTL 3). In addition, a method is disclosed in which a casing or part of the casing that constitutes the nozzle path is rotated about the core at a high rate.
In addition, a plasma torch provided with a rotary head having an eccentrically disposed plasma nozzle is disclosed (for example, refer to PTL 4).
Although not aiming to treat a large area within a short period of time, a high-speed gas shield are welding method, characterized in that band-shaped electrodes are used, and disposed so that the width direction forms the welding line direction and welding is performed, is disclosed as a welding method using the thermal plasma (for example, refer to PTL 5).
In addition, an induction coupling-type plasma torch provided with a plasma chamber composed of a flat rectangular insulating material having a linear thin and long shape is disclosed (for example, refer to PTLS 6 and 7). A plasma flood gun of an ion implantation system which includes a plasma chamber having a rectangular space is disclosed (for example, refer to PTLS 8 and 9).
Meanwhile, a method of generating thin and long linear plasma in which long electrodes are used is disclosed (for example, refer to PTL 10). Although described to generate thermal plasma, the method is to generate low-temperature plasma, and is not a configuration appropriate for a thermal treatment. If the thermal plasma is generated, since the method is a capacity coupling-type in which electrodes are used, it is assumed that an arc discharge is focused at one place, and it is difficult to generate uniform thermal plasma in the longitudinal direction. Meanwhile, as a low-temperature plasma processing apparatus, an apparatus with which a plasma processing such as etching or film formation is possible by plasmatizing etching gas or chemical vapor deposition (CVD) gas is used.
In addition, a method in which long plasma is generated using a micro strip line is disclosed (for example, refer to PTL 11). In this configuration, since the chamber wall surface with which the plasma comes into contact may not be completely cooled (not surrounded by a water cooling path), it is considered that the configuration may not work as a thermal plasma source.
In addition, an apparatus in which a plurality of discharge electrodes are arrayed linearly so as to form a linear long plasma torch is discloses (for example, refer to PTL 12).